The present invention relates to a system for transmitting and/or receiving signals and more particularly to a system of antennas for tracking nonsynchronous satellites.
Hitherto, commercial telecommunications via satellites have been achieved almost entirely via geostationary satellites, which are especially beneficial by virtue of their unchanging relative positions in the sky. However, the geostationary satellite exhibits major drawbacks such as considerable attenuations related to the distance separating the user antennas from the geostationary satellite (of the order of 36,000 kilometers, the corresponding losses then rising to around 205 dB in the Ku band) and transmission lags (typically of the order of 250 ms to 280 ms) thus becoming clearly perceivable and perturbing especially for real-time applications such as telephony, video conferencing, etc. Furthermore, the geostationary orbit, situated in the equatorial plane, poses a visibility problem in respect of the regions at high latitudes, the angles of elevation becoming very small for the regions close to the poles.
The alternatives to employing geostationary satellites are:
the use of satellites in inclined elliptical orbits, the satellite then being almost stationary above the region situated at the latitude of its apogee for a duration of possibly up to several hours,
the implementation of constellations of satellites in circular orbits, in particular in low orbit (xe2x80x9cLow Earth Orbitxe2x80x9d or LEO) or in mid-orbit (xe2x80x9cMid Earth Orbitxe2x80x9d or MEO), the satellites of the constellation flying past in turn within visibility of the user terminal for a duration of from some ten minutes to around one hour.
In both cases, service cannot be provided permanently by a single satellite, continuity of service demanding that several satellites fly over the service area one after another.
The document U.S. Pat. No. 4,531,129 describes a Luneberg lens scanning antenna system which includes several sources. However, this system is too complex.
The aim of the invention is therefore to produce a system of antennas for tracking satellites, making it possible to pick up at least two nonsynchronous satellites following one another within the area of visibility of the system and remedying the drawbacks of the prior art systems.
To this end, the subject of the invention is a system for transmitting and/or receiving signals in a communication system employing nonsynchronous satellites, comprising pluridirectional focusing means possessing a focusing surface including a plurality of focal points, comprising a first signals transmitter and/or receiver element and a second signals transmitter and/or receiver element, first means of support of the first element and second means of support of the second element, which means of support are independent of one another and arranged respectively along a first and along a second continuous line of focal points, characterized in that the said system furthermore comprises:
a first means of displacement of the first element along the first continuous line of focal points and a second means of displacement of the second element along the second line of focal points on the first and second support means respectively,
means of control of the first and second means of displacement for displacing the first and second elements along the said first and second line of focal points.
In this way, the system according to the invention makes it possible to transmit and/or pick up at least two spatially separate beams and to not suffer from a switching lag when switching from a first satellite to another satellite, which lag would be due to the repositioning of the reception means which sighted the first satellite to a position sighting the second satellite. In the present patent application, the term xe2x80x9cactivexe2x80x9d will be ascribed to any element which exchanges at least a major part of the useful data with a likewise so-called xe2x80x9cactivexe2x80x9d satellite. The term xe2x80x9cpassivexe2x80x9d will designate any other element which exchanges signaling data and little useful data with a so-called xe2x80x9cpassivexe2x80x9d satellite. Since the means of support are independent, the first and second elements can move along the focusing surface without perturbing one another.
According to one embodiment, the system according to the invention comprises means of monitoring the control means so as to determine the transmitter and/or receiver element with which the useful data exchanges are to be performed and so as to switch the latter element to means for processing the signals received and/or transmitted and which are linked to an outside unit for utilizing these signals.
According to one embodiment, cyclically, during a determined period, the two elements are switched to the said processing means.
According to one embodiment, the monitoring means are included within a fixed device of the system.
So that the displacement of the first and second elements should not be disturbed by the presence of cables, the said first and second elements respectively include first and second antennas for receiving and/or transmitting signals and the said device comprises third and fourth transmission and/or reception antennas and in that, while operational, the said first and second elements and the said device are linked by a wireless radioelectric link.
According to one embodiment, the said support means are arranged on either side of the focusing surface.
According to one embodiment, the said focusing means comprise a spherical Luneberg-type lens.
According to one embodiment, the said support means comprise semicircular rails whose centers of curvature almost coincide with that of the lens and are coupled to means of actuation of the said support means.
According to one embodiment, the said actuation means include means of rotation of the first and second support means for azimuthal tracking of the satellites.
According to one embodiment, these means of rotation comprise a rotation shaft passing through the center of the Luneberg lens and around which the said first and second support means are able to revolve.
According to one embodiment, the said first and second support means include rails and in that the said first and second elements are provided with motors enabling them to be moved on the said rails.
According to one embodiment, the monitoring means control the motors of the elements and the means of actuation of the rails.
According to one embodiment, the said first and second elements comprise printed-circuit plate antennas.
According to one embodiment, the said first and second transmission and/or reception elements respectively comprise a frequency converter block for the transmission signals and/or a frequency converter block for the reception signals.
According to one embodiment, the system according to the invention furthermore includes at least one third transmitter and/or receiver element located in the vicinity of one and the same focal point of the system for communication with at least one geostationary satellite.
According to one embodiment, the system according to the invention is intended for tracking nonsynchronous satellites along predefined trajectories and in that the said first and second lines of focal points correspond to the trajectories of a first and a second satellite.
In the present patent application, the term xe2x80x9cdown signalxe2x80x9d shall be interpreted as a signal conveyed in the direction from the satellite to a unit inside a dwelling to which the system is linked, whereas the term xe2x80x9cup signalxe2x80x9d will relate to a signal conveyed in the direction from the unit inside the dwelling to the satellite.